The present invention relates to a novel extruder which contains a screw which has separate, dual flights thereon whereby two separate thermoplastic materials can be separately extruded in a single extruder to give a product having separate co-extruded layers of thermoplastic materials.
It is common in the art to co-extrude multiple layers of thermoplastic material to provide multilayered products such as pipe and sheet. The conventional equipment for co-extrusion comprises two or more extruders feeding multiple channels of a single die or tool. See, for example, U.S. Pat. No. 3,649,143.
By use of the presently disclosed invention, a single extruder feeds the tool or die, thus eliminating the costs of multiple extruders. The extruder screw of the present invention is provided with two or more flights, each being fed by separate hoppers, and each delivering thermoplastic separately to the die of the extruder.
In order that there be a single conveyance of the material through the extruder, there is substantially no clearance between the flight of the screw and its confining chamber or barrel, and the screw must have a substantially constant pitch and root diameter. Two separate sets of flights, each being fed separately, are used to convey and melt the thermoplastic materials and feed them to an extrusion die. The thermoplastic materials maintain their own distinct identity in the extruder until they are fed separately to the die for co-extrusion.